It is difficult to make safe and accurate measurements of electrical voltage signals that are elevated at a voltage significantly above a ground potential. Quite often, the signal being measured is a low-level signal riding on a high-level common-mode signal, requiring the measuring device to have a high common-mode rejection ratio. Moreover, extraneous ground currents may add hum and ground loop induced voltages to the measured signal. These problems are particularly troublesome when making wide bandwidth oscilloscope measurements of electrical voltage signals.
The above-described measurement problems frequently force oscilloscope users into employing potentially dangerous measurement techniques, such as "floating" the oscilloscope by defeating its ground reference connection. Typically, the oscilloscope is floated by using an isolation transformer, a 3-to-2 prong power plug adaptor, or by simply cutting off the grounding prong on the oscilloscope power plug. Floating the oscilloscope not only creates a high voltage shock hazard on the oscilloscope, but also unduly stresses its power transformer insulation and reduces measurement accuracy because of capacitance-induced ground current.
Clearly, for safe and accurate measurements, an oscilloscope should be connected to ground. Fortunately there are several safe and accurate floating measurement solutions available including indirect grounding devices, differential measurement devices, isolated input systems, and isolation amplifiers.
Indirect grounding devices are connected between the measuring device and the power main. An exemplary indirect grounding device is the model A6901 Ground Isolation Monitor, manufactured by Tektronix, Inc., Beaverton, Oreg., the assignee of this application. The Ground Isolation Monitor allows the measurement device to float up to a safe signal reference level of about 40 volts and immediately reconnects the measurement device to ground when the signal reference level exceeds 40 volts. Unfortunately, measurements are limited to the 40-volt limit, the entire measurement device is elevated at the reference level, inadvertently grounding the measurement device can short circuit the measured circuit, and a significant capacitance between reference ground and power ground can cause inaccurate measurements.
Differential voltage measurements that do not require floating the measurement device can be made with high accuracy by employing a differential amplifier, such as the model 11A33 manufactured by Tektronix, Inc., Beaverton, Oreg., the assignee of this application. Unfortunately, differential amplifiers are costly and complex, have limited common-mode rejection, require pairs of balanced probes to connect to the signal being measured, and are limited to measuring signal voltages below about 500 volts.
Isolated input systems include battery-powered, hand-held measuring devices that are inherently isolated from the power mains. Such devices are usually well insulated and are capable of measuring small signals elevated by hundreds, or even thousands, of volts. Unfortunately, battery-powered measuring devices typically have limited measurement sensitivity and bandwidth and can present significant capacitance to the circuit being measured.
Isolation amplifiers are typically connected between a circuit being measured and the measuring device. An exemplary isolation amplifier is the model A6902B Voltage Isolation Amplifier, manufactured by Tektronix, Inc., Beaverton, Oreg., the assignee of this application. Isolation amplifiers provide an isolation barrier across which an input signal being measured is coupled. Isolation amplifiers offer a preferred solution to floating measurements because the input signal may be elevated at a voltage of up to about 3,000 volts, the capacitance across the isolation barrier is relatively low, and the measurement bandwidth spans a range from zero Hertz (direct current, or "DC") to about 25 MegaHertz because a split-path isolation technique is employed.
The split-path isolation technique employs separate isolation barrier devices for coupling a DC-to-low-frequency ("low path") component and a low-frequency-to-high-frequency ("high path") component of the input signal. Outputs from the low path and high path are recombined by a summing amplifier to generate a resultant, DC to high-frequency ("wideband") output signal.
The A6902B high path and low path employ respectively a transformer and an optocoupler to couple separate frequency components of the input signal across an isolation barrier to drive the summing amplifier. Linearity of the low path is controlled by a second optocoupler that provides a closed-loop feedback path. Unfortunately, low path linearity depends on the degree to which the thermal and electrical transfer characteristics of the two optocouplers are matched. Linearity of the high path depends on the coupling characteristics of the transformer, which depend to a large degree on the magnetic flux characteristics of it core material. The above-described linearity determining factors co-act such that the A6902B requires 17 adjustments to properly balance its gain, offset, and frequency response.
Measuring electrical signal currents typically requires inserting a current measuring sensor in the current carrying conductor, an operation that is potentially hazardous, alters the frequency response of the conductor, and leads to inaccurate high-frequency measurements. As a result, prior workers have employed Hall-effect devices to sense current flow-induced magnetic flux in the conductor to measure DC-to-medium-frequency signal currents without breaking or electrically contacting the conductor. It follows that a Hall-effect device could be employed in the low path of an isolation amplifier if the signal voltage was first converted to a signal current. An exemplary current sensing Hall-effect circuit is described in U.S. Pat. No. 3,525,041 issued Aug. 18, 1970 for MAGNETIC FIELD MEASURING METHOD AND DEVICE EFFECTIVE OVER A WIDE FREQUENCY RANGE, which is assigned to the assignee of this application. Unfortunately, Hall-effect devices have a current sensitivity that varies with temperature, thereby requiring frequent calibration to maintain measurement accuracy. In addition, Hall-effect devices are not well-suited to employing feedback techniques and, therefore, have relatively poor linearity. The same has been generally true for optocouplers, which are used extensively for coupling and isolating digital signals where signal fidelity is not an important factor. Prior split-path isolation amplifiers also have difficulty maintaining signal accuracy at a crossover frequency region in which the low-path and high-path frequency ranges overlap. It is well known to match the 3-db low path and high path rolloff frequencies to achieve a crossover frequency, which causes a slight "dip" in the amplitude of the combined signal at the crossover frequency. Therefore, conventional split-path amplifiers typically employ a dozen or more variable resistor and capacitor adjustments to correct the combined signal amplitude in the crossover frequency region. The variable resistors and capacitors must be precision components that unfortunately add significant cost to prior isolation amplifiers, reduce their reliability, and require periodic readjustment to maintain signal accuracy.
What is needed, therefore, is a simplified linear split-path isolation amplifier that eliminates frequency compensation adjustments, has broadened signal measurement bandwidth, reduced capacitance across the isolation barrier, and generally lowered cost and improved reliability.